Dual-Wavelength On-Chip Integrated Metalens for Epi-Fluorescence Single Molecule Sensing

Single molecule fluorescence spectroscopy offers unique capabilities of low concentration sensing or probing molecular dynamics. However, employment of such methodology for versatile sensing and diagnostics under point-of-care demands device miniaturization down to lab-on-a-chip sizes. Here, we numerically design metalenses with high NA of 1.1 that are composed of silicon nitride nanostructures deposited on a waveguide and may selectively focus guided light into an aqueous solution at two wavelengths of interest in a spectral range between 500 and 780 nm. Despite the chromatic focal shift in lateral directions owing to the wavelength-dependent propagation constant in a waveguide, a segmented metalens provides perfectly overlapped focal volumes which meets the requirements for epi-fluorescence light collection. We demonstrate that the molecule detection efficiencies for metalenses designed for excitation and emission wavelengths of fluorophores ATTO 490LS, Alexa 555, APC-Cy7 tandem are sufficient to collect several thousand of photons per second per molecule at modest excitation rate constants. Such sensitivity provides a reliable diffusion fluorescence correlation spectroscopy analysis of single molecules on chip to ex-tract their concentration and diffusion properties in the nanomolar range. Altogether, an achromatic integrated metalens opens new avenues for development of ultracompact and sensitive devices for precision medicine and environmental monitoring.